Measurement of the characteristics of a moving paper web is essential to the paper manufacturing process. A simple way to determine some of the mechanical properties of a material, such as a moving paper web, is to generate ultrasonic waves in the material at one location in a specimen and then to detect them at another point in the specimen at a given distance from the first location. The speeds of the ultrasonic waves along certain directions of the paper web will provide the rigidity tensor of the paper, which is related to the paper's mechanical properties. Numerous contact and non-contact measurement techniques exist and have varying degrees of success and application.
The direction of displacement of the surface of the paper caused by propagating ultrasonic waves is independent of the direction of propagation and can be in any direction in the plane of the paper (called in-plane displacements) and in any direction normal to it (out-of-plane displacements). By using a time base triggered at the instant of generation of the waves, the travel time between the generation and detection points can be measured. Using this travel time and if the precise distance between the generation and detection points is known, the speeds of the ultrasonic waves can be determined. The speeds of ultrasonic waves are related in a more or less simple way to some of the coefficients of the rigidity tensor of the paper, and thus are linked to some of the paper's mechanical properties. Measurements of the propagation speeds of specific ultrasonic waves by detecting both the in-plane and out-of-plane displacements can be used to evaluate some of the main mechanical properties of the paper and to control the paper machine.
Prior art methods of measurement of ultrasonic waves in various materials using laser interferometers can be found in the following references: "Broadband optical detection of ultrasound by two-wave mixing in a photorefractive crystal", R. K. Ing and J. P. Monchalin, Appl. Phys. Lett. Vol. 59, p. 3233-3235, 1991; "Detection of ultrasonic motion of a scattering surface by two-wave mixing in a photorefractive GaAs crystal", A. Blouin and J. P. Monchalin, Appl. Phys. Lett. vol. 65, number 8, August 1994; "Ultrasound detection on rough surfaces using heterodyne photorefractive interferometer: applications to NDE", R. K. Ing, D. Royer, Bruno F. Pouet and Sridhar Krishnaswamy, p. 681, 684, Proceedings of the 1996 IEEE Ultrasonics Symposium, San Antonio, Tex., November 1996; "Heterodyne interferometer with two-wave mixing in photorefractive crystals for ultrasound detection on rough surfaces", Bruno F. Pouet, R. K. Ing, Sridhar Krishnaswamy, D. Royer, Appl. Phys. Lett. Vol. 69, number 25, December 1996; "Paper stiffness monitoring using laser-ultrasonics", P. H. Brodeur, Y. H. Berthelot, M. A. Johnson, and J. P. Gerhardstein, Proc. 1996 IEEE Int. Ultrasonics Symp., San Antonio, Nov. 3-6, 1996; "Investigation of the mechanical properties of copy paper using laser generated and detected lamb waves", M. A. Johnson, Ph.D. Thesis, Georgia Inst. of Tech., 1996; and "Noncontact laser generation and detection of lamb waves in paper", P. H. Brodeur, M. A. Johnson, Y. H. Berthelot, and J. P. Gerhardstein, J. Pulp & Paper Sc. 23(5) J238-J243, 1997.
Several U.S. patents disclose techniques for measuring ultrasonic waves in materials in a non-contact way: U.S. Pat. No. 5,638,396, issued Jun. 10, 1997, to Klimek, entitled "Laser ultrasonics-based material analysis system and method;" U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin and Blouin, entitled "Generation and detection of ultrasound with long pulse lasers;" U.S. Pat. No. 5,585,921, issued Dec. 17, 1996, to Pepper, et al., entitled "Laser-ultrasonic non-destructive, non-contacting inspection system;" U.S. Pat. No. 5,131,748, issued Jul. 21, 1992, to Monchalin and Ing, entitled "Broadband optical detection of transient motion from a scattering surface by two-wave mixing in a photorefractive crystal;" U.S. Pat. No. 5,025,665, issued Jun. 25, 1991, to Keyes IV and Thompson, entitled "Non-contacting on-line paper strength measuring system;" and U.S. Pat. No. 5,680,212, issued Oct. 21, 1997, to Blouin, et al., entitled "Sensitive and fast response optical detection of transient motion from a scattering surface by two-wave mixing."
Measuring the properties of a moving web of paper requires overcoming several problems. First, the very nature of paper makes it difficult to use optical techniques. Because of the surface roughness of paper (from many fibers) and the roughness of the fibers themselves, the light reflected from the surface is scattered almost isotropically in a half space (i.e., the paper surface becomes a Lambertian source). As a result, the incident beam is backscattered by the paper forming a speckled reflection. While the power collected by a lens centered in the direction of the incoming beam will stay small, as long as the solid angle of collected light stays small, the speckled nature of the reflection makes it very difficult for all types of interferometers, working with only a single speckle, even on static surfaces, to detect ultrasound.
Piezoelectric transducers have been used to generate and detect ultrasonic waves on moving paper but have the disadvantage of either being contact transducers which could damage the product, or in the case of air-coupled transducers, offering poor energy coupling into the material, and thus a weak signal. Several non-contact ways of generating ultrasonic waves in materials exist, such as, air-coupled transducers or time-gated microwaves. A recent technique called Laser Ultrasonics is an interesting alternative to the use of piezoelectric transducers.
In Laser Ultrasonics, ultrasound is generated in the paper using the thermoelastic effect, which consists of impinging the surface of the sample with a laser spot for a very short time duration. The sudden thermal dilatation created by the absorption of the laser light by the material generates ultrasonic waves propagating in the paper. The generation of ultrasonic waves by a laser has the advantage of being a fully non-contact method.
Lasers are also interesting tools for detecting ultrasonic waves when considering the time coherence properties of laser sources. Indeed, interferometers using laser sources have been used to detect the high frequency and small amplitude displacements of the surface of the paper caused by propagating ultrasonic waves. U.S. Pat. No. 5,814,730 describes use of a laser beam and the Doppler effect to measure the orthogonal displacement velocity of an acoustical wave to determine various characteristics of a paper web.
The most commonly used prior art interferometers for the detection of ultrasound are the knife-edge interferometer, the Mach-Zehnder interferometer, and the Fabry-Perot interferometer. In each of these interferometers, the detection relates solely to the change of the properties of a laser beam by the ultrasound, so the detection is performed on the product without any contact.
Thus, appropriate generation and detection of ultrasonic waves are able to provide some of the mechanical properties of paper and by using laser, can be performed without any contact of any kind on the paper products.
While the above interferometers have been useful for measuring properties of static paper products, they have, however, some significant disadvantages when used on a moving paper web. Paper is an optically highly scattering material. The Mach-Zehnder and knife-edge interferometers have a small etendue (throughput), and thus, are very sensitive to the change of laser speckle pattern caused by the fluttering of a paper web. Because these two interferometers work mainly with the specular reflection of laser light, it is expected that the reflected beam carrying the ultrasonic information would be unstable, when impinging fast moving paper. Thus, most single speckle interferometers such as Mach-Zehnder, Michelson, Sagnac, polarimetric and self-mixing interferometers need to use special, intricate and delicate devices such as a scanning mirror, time triggering of the ultrasound generation, etc. to detect ultrasound on surfaces that scatter light a lot such as paper. However these solutions provide new problems: having to use moving parts and the sensitivity of the mirror to vibrations. So these interferometers are not really acceptable for moving paper webs.
The Fabry-Perot interferometer provides the advantage of having a large etendue, and thus is able to work on very scattering surfaces and on materials moving at a high speed. Unfortunately, its bandwidth provides an acceptable sensitivity only for frequencies above 1-2 MHz. The frequencies of the ultrasonic waves traveling in paper are typically in the range of 20 kHz to 2 MHz, due to paper's properties as an attenuating lowpass filter. Thus, when used on moving paper products, none of the above interferometers would be really satisfactory.
There is a need for a non-contact system and method of measuring ultrasonic waves on a moving paper web. There is a need for a system and method employing an interferometer with high etendue. There is also a need for a system and method which can measure both in-plane and out-of-plane displacements. There is a need for a system and method of measuring ultrasonic waves on a moving paper web in real time.